solar battery 1 or bank?

[GypsyDogs]

ok.. dumb question
with my limited budget/space/weight

which is a better option?

100 ah 12v agm x1  (~$200)
55-80 ah 12v agm x2  (~$150 ea=$300)

or, painfully$ but allegedly better
180 ah 6x amg x2 (~$400-450)

looking at having about 150 watts of solar on a little victron controller.

[B and C]

More is almost always better.  The big question is what is your daily amp usage?  From what I understand, shallow discharging leads to longer battery life.  My 350 Ah AGM bank is almost 9 years old.  I will probably have to replace it later this year or early next year.  I only use about 40 Ah's over night.  This gives me reserve for those bad solar charge days and I have a generator if it really gets bad.

SW hopefully will weigh in.  I know he gets the maximum out of his batteries. 

[GypsyDogs]

I am pretty scosh on battery usage..
that is charging phone, lights, and dometic 35, and occasional 12v fan usage.
Had 160 in the van and barely touched it. - usually overnight state was 12.48 - 12.52
And I know better now- how to retain cold in my fridge..
And plan to do any phone/laptop charging only while pulling down from the great battery in the sky.. lol

have a victron 75/15 mppt

[sternwake]

I am not of the opinion that more battery capacity is always better, but I am of the opinion that more solar is.

There are so many variables and no one right answer, but in general the higher the solar wattage to battery capacity ratio, the happier and longer lived the battery will be, and this is more true with AGM batteries.

For 150 watts of solar and solar being the primary charging source flat on the roof and not aimed at the sun for max harvest, I would not want more that 75 AH of battery capacity, and that would be in a sunny environment. I'd have no fear of using upto 55 of those AH here and there, but not each and every night.

really AGMs when they are depleted to 50% or below should receive no less than 20% of their Ah capacity, 20 amps per 100 Ah o capacity, and this is difficult to achieve via solar.

There can be two strategies, more capacity discharged lesser amounts, or lesser capacity charged at higher rates.

My opinion and personal strategy is to deeply cycle lesser capacity and have the ability to feed the high quality AGM battery high amperage via my Alternator, or plug in charging source, and the Solar is basically able to hold the battery at absorption voltage for as long as it takes for amperage, when held at absorption voltage, to taper to 0.5% of capacity.

Since most newer to living on 12vdc will not understand the importance ad difficulty of holding the battery at absorption voltage as long as required to reach a true full state of charge, and just how much better that reaching 100% is compared to reaching 98%, then a lesser amount of capacity also hurt$ less to replace prematurely/ more often.

So I would say is 150 watts the maximum you can put on your roof?

What if any other charging Sources are you going to employ?

If you are going to employ alternator charging, do you have any idea of the voltages your vehicle allows? The voltage it allows is more important than its Amperage rating of the alternator, as if it only seeks low voltage then high amps cannot flow into a depleted battery. Honestly alternator charging depends a LOT on this voltage, and most vehicles voltage regulation is very timid, more concerned with never coming close to overcharging the battery. Usually only a short period is allowed with higher electrical pressure, where it can allow the depleted battery to suck larger amperages before reverting to the 'safe' float voltages that are designed to prevent discharging while driving and not coming close to overcharging. There are a few solutions to the imperfections of alternator charging, but none of them or cheap or simple or both, and it varies a lot depending on vehicle platform.

6v AGM batteries do NOT have the advantage that6v AGM batteries have over their 12v counterparts. 6v AGMS are basically made to replace flooded 6v batteries in a batteyr compartment designed for 6v batteries.

If one wants 100Ah of 12v battery, it is better to have one 100AH battery than 2 50Ah batteries in parallel.

Battery Voltage is not to be considered like a fuel gauge. It can give indications as to state of charge of the battery, but it is not
set in stone as to the state of chrge and certainly not to be indicative of the state of health of the battery. Longer times of observation of the specific battery in use and the voltages it retains under loads will allow one to understand the specific battery better, but once sulfation starts from undercharging the numbers seen when new will not bee seen later on, and even once the battey is indeed fully charged to its maximum remaining capacity the new to older voltage readings under load/in use can not be compared directly.

As always achieving ideal battery longevity can be taken to ridiculous extremes, and there is no one right answer, and lots of Misinformation out there, and lots of simply Bad advice by those with very incomplete understandings of how lead acid batteries want to be treated. One pesons 'good enough' can be far from someone elses, and the seemingly universal human need to prove just how smart they are will have them make unsupportable unverifyable and physics defying claims.

More battery capacity gives some the warm and fuzzies, but when that capacity is actually used, the recharging of that extra battery capacity requires more charging ability. higher amperage and holding absorption voltag becomes even more important.

I have gone from as much as 345 total Ah capacity, for house and engine startung battery, to just 90AH now for both engine and house. My AGM battery is now over 5 years old and While it is definitely showing its age, when cranking the engine, it is not showing its age, voltage retention wise when overnight discharging. It is taking much much longer for amps at absorption voltage to taper to 0.5% of capacity, making it more important for my solaror other chrging sources to get the battery to absoprtion voltage earlier in the day, meaning more solar would be more beneficial than if I were cycling more capacity, as it is time that I need,and shorter winter days are shot on time, so more wattage to achieve VABS earlier insures more time to reach a true 100% state of charge.

98% charged is good, no one is going to notice in one cycle the difference between the battery that was 100% chrged, vs the one which was 98% charged, but after a week of 98% only charges, compared to 100% charges attained, the 100% charged battery will perform better, and its capacity will degrade at a muh much slower rate.

Whether you choose less solar to replenish more battery capacity that was discharged less, more lesser battery capacity with a higher wattach to capacity ratio, cycled deeper, well vigorous arguements can be madeon both sides.

For me and my Situation and usage and charging sources abilities, I am getting by far the best service from lesser capacity than I did from more capacity, and my Van is carrying around 120 less Lbs of lead to do so.
That said, my 345Ah of flooded marine battery was not really recharged as well as I could achieve now, as back then I did not understand ow long it actually took at absorption voltages to achieve a true full charge.

Being able to plug in for 24+ hours with a charging source which can seek and hold absorption voltages has been key to my current battery's longevity. But I also cycle it as deep as I want kowing that I can plugin when many days have gone by without either the higher amp blast my alternator can achieve, or when the solar has not had enough sun or time to achieve 100% a few days in a row.

Are you going to be able to plug in for 24 to 36 hours every so often and have a relatively good charger/converter hold the battery at higher voltages until amperage tapers to 0.5% of capacity or less every so often? If one can and does do this, then one can get away with more battery abuse, but more charging ability is always battery. I can;t say the same about more battery capacity, especially when there is not the ability to perform the longer extended charge after many partial state of chrge cycles are accumulated on the battery.

Shoot for a 2 to 1 or higher solar wattage to Ah capacity, especially for AGM, and 3:1 would be better, in my opinion and experience.

[GypsyDogs]

I can readily handle up to 200 watts.. with the controller I have.

200 watts will fit comfortably on my roof.
No easy alternator charging.
Periodic ability to plug in and use one of these- https://www.walmart.com/ip/Schumacher-15...r/56303122 is doable.

[GypsyDogs]

More accurately, I have 200w and the Vic 75/15 controller...

So you are saying my battery would be happier identifying as a 75-100ah 12v.

(I live in Eugene.. so framed the question in a voltage neutral way.  .. gotta be PC here.. lol)

[tx2sturgis]

I agreee with Sternwake.

Especially for flat, roof mounted, non-tiltable panels, a 2 to 1 ratio should be considered a minimum wattage to ah balance if boondocking and powering a fridge.

I have 200 watts flat solar to 110 ah AGM, (PWM controller) in my truck camper and it is just getting by, with low sun angles and several cloudy days. 

(This is an 'experiment' of sorts, running a modest system and an un-proven combination of inverter and fridge.) 

On sunny days it is performing quite well, there is a surplus, but you should try to spec the hardware for non-optimal conditions.